Locusts are insects that look a bit like big grasshoppers. They're a classic example of what's called phenotypic plasticity. An organism's phenotype is basically its observable characteristics--behavior, color, size, etc. (Phenotype is the outward expression of genotype; genotype is the specific group of alleles that an organism has. Most--all? I'm not sure--genes have at least two alleles, or "flavors." The classic example is, of course, Mendel's peas; the gene that controls flower color in pea plants has two alleles, purple and white. A pea plant's flower-color genotype is the particular combination of alleles that it has; its flower-color phenotype is the particular color of flower it produces.)
Organisms such as locusts that demonstrate phenotypic plasticity can undergo significant changes in behavior, appearance, etc due to changes in their surrounding environment. Locusts are a classic case because the change is so dramatic. If you take two locusts and put them in a box, they will pretty much avoid each other--that is, assuming they're demonstrating the "solitarious" phenotype. This is pretty much the default position for locusts; most of the time, they hang out by themselves (not a lot of singles bars in their neighborhoods, I guess).
Now if you had put, say, 20 or 30 solitarious locusts in that box and shut them in for a couple of hours, they would be quite changed when you opened the box. They would be swarming together, and they would have changed in appearance (from kind of boring and green to a rather striking, Steelers-like combination of yellow/tan and black...sorry, couldn't help it). They would be demonstrating the "gregarious" phenotype.
Solitarious (top) and gregarious (bottom) desert locusts. Image from Dr. Tim Matheson, University of Leicester
Locusts in the gregarious phase are the stuff of legend. These are the critters that mow crops down to the roots and blacken the skies. (Presumably, the Egyptians crowded their locusts.)
The cause of the transition from solitarious to gregarious has been known for a while, at least in broad strokes: being in the presence of lots of other locusts makes a locust more gregarious. In a recent article in Science, Anstey et al identify the mechanism that triggers the transformation.
There are two different sets of stimuli that can make a locust more friendly: mechanical and "cephalic." Mechanical stimulation involves being jostled by other locusts; in contrast to most humans, most locusts become more friendly when strangers stroke their legs. Cephalic stimulation involves the sight and smell of other locusts; locusts apparently have really great makeup and cologne. Both types of stimuli cause the locust's central nervous system (CNS) to produce (what else?) serotonin. (Yes, that serotonin.)
Previous researchers established that serotonin levels are higher in locusts undergoing the solitarious-to-gregarious transition. Anstey et al set out to determine the limits of this relationship. They did four main experiments: first, they figured out whether artificial stimulation of the individual sensory pathways could stimulate serotonin production and gregariousness. Then, they tested whether serotonin antagonists (i.e., chemicals that block the action of serotonin) could prevent the onset of gregarious behavior. Third, they determined whether artificially increasing serotonin levels was enough to induce gregariousness. Finally, they determined whether giving the locusts a serotonin precursor (i.e., a chemical that is easily converted to serotonin) increased their sensitivity to environmental stimuli.
In the first experiment, the researchers either stroked the hind legs of solitarious locusts, stimulated the nerve connecting the legs to the CNS directly, or put the locusts in a cage that allowed them to see and smell (but not touch) other locusts. In all cases, the locusts switched from solitarious to gregarious, and serotonin levels increased, suggesting that either type of stimulation is sufficient to induce gregariousness.
Next, they injected some of the locusts with serotonin antagonists (they also, of course, injected others with just saline--this was the control group). After the injections, the treated locusts (the ones that received the antagonist) were significantly less responsive to stimuli than the control locusts; treated locusts did not become gregarious, even when exposed to stimuli that caused the control locusts to boogie down. This relationship showed that inhibiting the action of serotonin prevents the "phase change."
In the third experiment, Anstey et al applied serotonin directly to the locusts' nerves (again, they also used a control group that received just saline). They also injected a third group of locusts with a serotonin agonist (i.e., a chemical that increases the activity of serotonin--the opposite of an antagonist). The treated locusts became much more friendly, but the control locusts remained aloof. In other words, just increasing serotonin levels (without actual stimuli) can make solitarious locusts more gregarious.
In their final experiment, the researchers determined whether increasing the ability of the locusts to produce serotonin would cause them to become more gregarious after only a small amount of stimulation. Typically, a solitarious locust has to hang out with other locusts for a couple of hours before putting on its party shoes. However, when solitarious locusts were injected with a serotonin precursor, 30 minutes of exposure was enough to get them dancing.
There's some hope that these results might lead to new possibilities for locust control. Individually (i.e., in the solitarious phase), locusts aren't too much of a problem--no more so than, say, grasshoppers, really. It's only when they start to swarm that they become economically disastrous. If a way could be found to prevent locusts from become gregarious, even when crowded, then locust swarms could be controlled. (Too late for Rameses, of course.) Such possibilities are still in the future--currently, there is no locust-specific serotonin antagonist that can be applied appropriately--but it does give some hope.
Until then, there is one thing we can definitely conclude: keep the locusts away from the Prozac!
Anstey, Michael L., Stephen M. Rogers, Swidbert R. Ott, Malcolm Burrows, and Stephen J. Simpson, 2009. "Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts." Science 323: 627-630. doi 10.1126/science.1165939
Stevenson, P.A., 2009. "The key to Pandora's box." Science 323: 594-595. doi 10.1126/science.1169280
(Yes, I know this isn't quite 1,000 words. But it's pretty darned close!)